Fuse for Bucky???

[jjasilli]

How to fuse a bucking transformer?   Schematic attached.  Tranny is Radio Shack 6.3 - 0- 6.3 @ 1.2A.

Finally getting around to rebuilding my Traynor Bassmate YBA-2B, EL84 model.  To get plate voltage down to 300V (from 409V), one thing I did was to mount and wire a bucking tranny. Works great! The UL rating for the amp states that it draws 1A. The amp uses a 2A slo blo fuse.

I wired the bucking tranny before the stock fuse.  It occurs to me that there could be a hazard if the bucking tranny shorts out, as it itself is not fused.  Is all OK as-is per the schematic? Should the 2A fuse be moved to in-front of the bucking tranny?  Should a second 2A fuse be installed before the bucking tranny?   Thanks!

[sluckey]

Should the 2A fuse be moved to in-front of the bucking tranny?

That's what I'd do. One fuse is enough. I actually like to tie the line cord directly to the fuse too.

[RicharD]

Agreed but I'd put the fuse in front of the switch.  What happens if you break the switch?  The IEC socket is a disconnecting means.

[jjasilli]

Thanks guys!

[jeff]

What exactally is a bucking transformer? You said you're using this to get the plate voltages down to 300V from 409V, are you also lowering the heater voltages?

[jjasilli]

A step-down tranny is used.  The secondary voltage is fed back to the wall voltage, out of phase, to subtract from, or buck, the wall voltage.  See:  http://sound.westhost.com/articles/buck-xfmr.htm and especially http://geofex.com/ > Vintage AC adapter

This is much like using a variac, except the step-down is fixed, not variable.

Yes, heater voltage is the limiting factor.  A nominal 6V heater can safely run on 5V (see Merlin's Valve Wizard site).  Let's say 5.3V for a margin of error; but is usually fed 6.3V:  5.3/6.3 = .84  So that's the largest factor that we can use to safely reduce supply voltage. Most PT specs assume 115VAC wall voltage X .84 = 97VAC.  So that's the lowest voltage we want the bucking tranny (or a variac) to put-out.  My wall voltage is actually about 123VAC.  The 12.6V tranny should buck that down to about 110VAC.  I'm actually getting 109VAC, well above 97VAC. So the math predicts the heaters will be safe.  In fact, my DVM reads 6VAC on the heaters.

Also, I said one thing I did was use the bucking tranny.  The other thing I didn't mention is a 330Ω 25W inductive dropping resistor in the B+ rail.  Under load, the bucking tranny and the power resistor drop about 50 plate volts each from the stock voltage to produce the overall 100V drop in the plate voltage.  

I'm still toying with little bottle voltage downstream.  This amp uses a voltage divider --  22K series/150K shunt -- in the B+ rail to drop voltage to the PI & preamp.  Removing the shunt resistor restores PI & preamp plate voltages to stock values.